There are a number of scenarios in mobile communications where the desired cell structure and the desired number of cells are time-dependent. For instance, some parts of a mobile communications system may experience a high load during daytime and a lower load at night. This means that the resource requirement can be drastically different over the course of 24 hours.
Similarly, the long term average load in a mobile communications system will typically increase over time, which means that the overall load in a particular area will change. The system will then have to be reconfigured to incorporate additional resources, for example as realized when increasing the number of cells.
Examples of antenna- and propagation-related solutions to increase load capacity are higher-order sectorization and addition of new sites, both solutions providing an effective cell split.
The solutions above are non-reversible in the sense that once they are deployed, the system complexity and resource allocation is permanently increased. There are no non-trivial ways to reverse cell split using conventional base station configurations.
U.S. Pat. No. 6,091,970 discloses a base station comprising an arrangement of several directional antennas whose individual azimuthal beam patterns achieve a substantially omnidirectional coverage. In one illustrated embodiment the signal transmitted from one base station transceiver is split in three signals which are fed to an antenna configuration of three directional antennas so as to provide an almost omnidirectional or “pseudo-omnidirectional” pattern. All antennas in the antenna configuration use the same polarization for transmit and receive and an additional diversity receiver is using a different polarization. The main drawback with this solution is that a number of sharp null-depths are created in the “pseudo-omnidirectional” pattern which will cause areas of poor or no coverage. The U.S. Pat. No. 6,091,970 includes phase shifters whereby two of the transmitted signals can be shifted in phase. However this solution only moves the interferometer pattern resulting from the combined radiation pattern from the three antennas. This means that the null-depths are moved but not eliminated. There is a need to avoid the problem with interferometer pattern causing null-depths that occurs when antenna patterns with the same polarization are combined.
The effect of the phase shifters in U.S. Pat. No. 6,091,970 only works over a limited bandwidth which means that the solution also has the disadvantage of being narrowband. As the phase shifters are inserted in the output lines the phase shift effect only works for the transmitted signals, i.e. it is a downlink solution only.
U.S. Pat. No. 6,577,879 B1 describes how an antenna pattern control is maintained by employing orthogonal polarization orientation for every other beam. An advantage with the present invention over U.S. Pat. No. 6,577,879 B1 is that it provides a solution also to the problem of providing a combined, omnidirectional radiation pattern without null-depths when employing a solution with an odd number of beams from directional antennas where each beam is covering an angular sector of a full 360° omnidirectional coverage.
There is thus a need for an improved, reliable and low complexity solution that eliminates the drawbacks of the existing solutions.